The two superpowers divided by Syria’s civil war met head to head on Thursday (NZT Friday), with signs emerging that Russia might curb its support for President Bashar al-Assad and Washington saying events were gathering speed on the ground.

US Secretary of State Hillary Clinton met Russian Foreign Minister Sergei Lavrov in Dublin on the sidelines of a security conference, at a time when rebel advances have brought the 20-month war to the doorstep of the capital Damascus.

‘‘Events on the ground in Syria are accelerating, and we see that in many different ways,’’ Clinton said before the meeting.

‘‘The pressure against the regime in and around Damascus seems to be increasing.’’

Lakhdar Brahimi, the UN mediator who convened the meeting, said the two sides had not made any breakthroughs, but would seek a ‘‘creative’’ solution to their impasse.

Washington and its allies have long sought the overthrow of Assad, while Russia has shielded him at the UN Security Council.

But comments by an ally of President Vladimir Putin indicated Moscow may be losing patience with Assad.

Western countries and international officials have issued coordinated warnings in recent days to Assad not to use chemical weapons.

In the latest, the United Nations said Secretary-General Ban Ki-Moon had written to Assad to urge him not to use poison gas.

US officials said they have seen intelligence indicating Assad might be more likely to use chemical weapons, although they have been vague about the nature of the information.

‘‘I think there is no question that we remain very concerned, very concerned that as the opposition advances, in particular on Damascus, that the regime might well consider the use of chemical weapons,’’ Defense Secretary Leon Panetta said.

Syria has not signed an international chemical weapons treaty banning poison gas, but has repeatedly said that it would never use such weapons on its own people.

Assad’s government said the warnings about chemical arms were aimed at whipping up an excuse for military intervention.

Nato decided this week to send US, German and Dutch batteries of air-defence missiles to the Turkish border, meaning hundreds of American and European troops will deploy to Syria’s frontier for the first time since the war began.

‘‘Syria stresses again, for the 10th, the 100th time, that if we had such weapons, they would not be used against its people.

‘‘We fear there is a conspiracy to provide a pretext for any subsequent interventions in Syria by these countries that are increasing pressure on Syria.’’

As darkness fell in the embattled capital, the highway to Damascus international airport was closed by fighting, witnesses said.

Rebels said they would not storm the airport, but would encircle it to stop flights supplying the army.

Fighting has intensified around the capital in the past week, prompting Western commentators to speak of an ‘‘end-game’’ that could soon see Assad toppled soon.

CONSTRUCTIVE MEETING

A senior State Department official said Clinton’s meetings with Lavrov and Brahimi were constructive, and US and Russian officials would follow it up with another meeting with Brahimi in coming days, seeking a political transition in Damascus.

Brahimi said: ‘‘We have agreed that we must continue to work together to see how we can find creative ways of bringing this problem under control and hopefully starting to solve it.’’

In one of the clearest signs yet that Moscow may be losing patience with Assad, Interfax news agency quoted the head of President Putin’s party in parliament as saying the Syrian government was no longer capable of funtioning.

‘‘We have shared and continue to share the opinion that the existing government in Syria should carry out its functions, but time has shown that this task is beyond its strength,’’ Vladimir Vasiliyev said.

Western countries have so far resisted conducting the sort of intervention in Syria’s civil war that saw Nato air strikes help topple Libya’s Muammar Gaddafi last year.

Germany approved the Patriot missile mission to Turkey on Thursday. Nato said it was a defensive step to prevent cross border strikes on alliance member Turkey, but Syria fears it could be a prelude to imposing a no-fly zone over its territory.

AIRPORT BLOCKADE

Rebel spokesman Abu Nidal said the army was pinned down along the airport highway by nightfall on Thursday (local time) by rebel fighters manoeuvring to mount a blockade.

The airport is not closed, but commercial traffic has almost ceased.

‘‘We know that arms have been going to the regime through the civilian airport,’’ he said.

A blockade would be ‘‘a good tool to put more pressure on the regime, which is part of strategy of trying to drain their strength’’.

Western powers have shown no enthusiasm for armed intervention in Syria, preferring economic sanctions, diplomatic pressure and limited aid to rebel forces, who get most of their guns and ammunition paid for by sympathetic Arab powers.

Britain said it would increase practical support for the rebels to include training and equipment such as body armour and night-vision goggles.

But they will not get the anti-aircraft and anti-tank missiles they are crying out for.

Exactly what Syria’s army has done with suspected chemical weapons to prompt a surge of Western warnings over the past two days is not clear.

Reports citing Western intelligence and defence sources were vague and inconsistent.

Clinton said on Wednesday Washington was concerned both about the possible use of chemical arms by ‘‘an increasingly desperate’’ Assad, and about the government losing control of such weapons to armed extremists - which could include rebels.

While Western countries support the rebel aim of toppling Assad, they were also uncomfortable with some rebel groups, which espoused radical Sunni Islamist views.

US officials said the Obama administration was considering blacklisting Jabhat al-Nusra, an influential rebel group accused by other rebels of indiscriminate tactics that had advocated an Islamic state in Syria and was suspected of ties to al Qaeda.

An explosion at the Damascus headquarters of the Syrian Arab Red Crescent killed at least one person on Thursday, Syrian state television said.

It blamed ‘‘terrorists from al Qaeda’’ — a term the government often employs to refer to rebel forces.

State news said a ‘‘terrorist’’ bomb exploded near a bakery in the Damascus suburb of Mezze 86, a district where many military families live and which has a large population of Alawites, the same sect as the president.

Opposition activists said army artillery pummelled several eastern suburbs of Damascus, where the rebels are dominant.

Suburbs had been cut off for weeks from water and electricity, rebels said, accusing the government of collective punishment.

Residents of the cosmopolitan capital — until now largely spared the ravages of a war concentrated in the provinces and other cities — spoke of a city under siege, with the sound of shelling becoming a constant backdrop.

Fighting was reported on Thursday in the rural outskirts of Damascus and in many parts of the country.

A crucial supply line for the army, the Damascus-Aleppo road, was hit by clashes.

Rebels said they have also surrounded an air base 4km from the centre of Damascus, a fresh sign the battle is closing in on the Syrian capital.